The invention relates to a switched-mode power supply comprising: an inductive element; a first electronic switch, coupled to the inductive element so as to cause current to flow through the inductive element during an on period in which the first electronic switch is conductive; and a parallel arrangement of a diode and a second electronic switch, also coupled to the inductive element so as to cause current to flow through the inductive element during an off period in which the first electronic switch is not conductive.
Such a switched-mode power supply is known, inter alia from International Application (PCT) No. WO 95/08863. Said known switched-mode power supply is of the flyback type, the inductive element being a transformer having a primary winding, which is connected to an input voltage during the on period and which is disconnected therefrom during the off period by means of the first electronic switch, and having a secondary winding, which powers a load via the diode and the second electronic switch which is arranged in parallel with said diode. When the first electronic switch is switched to the off state, i.e. during the off period, the diode is turned on by the voltage in the secondary winding and the magnetic energy built up in the transformer is supplied to the load.
When the output voltage across the load is comparatively small the voltage drop across the diode is comparatively large, as a result of which the efficiency of the power supply is reduced. The efficiency also diminishes for large currents through the load on account of the heat dissipation in the diode. In order to cope with this, it is known, inter alia from said International Application, to replace the diode by a second electronic switch, generally in the form of a MOSFET having a low forward resistance. This second electronic switch is turned on at the instants that the diode is turned on and is turned off at the instants that the diode is turned off. For safety reasons or for simplicity the diode is often not replaced by an electronic switch but the second electronic switch is arranged in parallel with the diode. In the case of a MOSFET the diode is often present anyway in the form of a body diode. The control of the second electronic switch requires a critical timing. For example, the second electronic switch should not be turned on until the first electronic switch has been turned off. If this is not the case, substantial switching losses may occur.
The second electronic switch can be controlled directly by means of a control signal which is in a fixed time relationship to the control signal of the first electronic switch. The known switched-mode power supply is an example of this. The control signals for the two electronic switches are the inverse of one another, fixed delay times being adopted to preclude overlapping of switching edges.
U.S. Pat. No. 4,870,555 discloses a forward converter in which the inductive element is connected between a voltage source and a load via an isolating transformer during the on period. In the off period the current flows through a synchronous rectifier. Said configuration requires two first electronic switches, one in series with the primary winding and one in series with the secondary winding, which should be driven simultaneously. The second electronic switch in the synchronous rectifier is controlled in phase opposition to the first electronic switches. A special logic circuit with cross-coupled NOR gates serves to prevent the second electronic switch from being activated prematurely.
European Patent Application EP 0,549,920 discloses still other configurations of switched-mode power supplies, in which the electronic switches in the synchronous rectifiers are driven into conduction briefly after the turn-on of the diodes arranged in parallel with these switches and are cut off briefly before the turn-off of the diodes. This requires a complex timing and control circuit.
Alternatively, the second electronic switch can be controlled indirectly by monitoring the voltage across and/or the current through the second electronic switch, a reversal of the sign of the voltage or the fact that a given current is reached being the indication to turn on or turn off the electronic switch. All these known control methods have the drawback that they require complex circuits and that their timing is critical.